follow: Fix args for $.post() call in toggleFollowingRepo()

Before, the resulting query string was _literally_ like:

?follows_repository_id=xxx&_session_csrf_secret_token=yyy

The server side stack ended up parsing it as:

>>> urllib.parse.parse_qsl("follows_repository_id=xxx&_session_csrf_secret_token=yyy", keep_blank_values=True)
[('follows_repository_id', 'xxx'), ('amp', ''), ('_session_csrf_secret_token', 'yyy')]

because the HTML encoding of & isn't relevant here and it thus looks like a
"matrix URL" (as drafted on https://www.w3.org/DesignIssues/MatrixURIs.html )
with the following values:

param: follows_repository_id = xxx
param: amp =
matrix: _session_csrf_secret_token = yyy

It thus ended up with the right values (if ignoring 'amp') and it thus worked
anyway.

Instead, clean it up and just pass a dict to jQuery.post as intended.